Differential detector circuits



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M. G. CROSBY DIFFERENTIAL DETEC'IIOR CIRCUITS Filed-July 17, 1941 Sept. 15, 1942.

`Af r g f aux@ K @hay BY 7 WV ATTORNEY Sept-,15, 1942- M. G. CROSBY A 2,296,092

- DIFFEREVNTIA'L DETEGTR CIRCUITS' .Filed July 1'7 1941 2 sheets-Sheet 2 ATTORNEY INVENTOR M607 6 1' BY i Patented' Sept. 15, `1942' Rossum 2,296,092' DIFFERENTIAL nurncron cnwurrs Murray G. Crosby, Riverhead, N. Y., miglior to America, a corporation o! Radio Corporation o! Delaware v Application July 17, 1941, Serial No. 402,784

l claims.

My present invention relates' to differential diode detector circuits adapted for reception of frequency, or phase, modulated carrier waves or amplitude modulated carrier waves, and more particularly to such detector circuits which include means for readily switching from one type of reception to the other type. l Y

In'the past the diiferential detector circuit ha beenconstructed to favor frequency modulated carrier wave (FM) reception, and for amplitude modulated carrier wave reception a separate detector was utilized, or a specic type of combining circuit was employed. Several methods have been employed to provide a detection circuit capable of utilization with either of the aforesaid types of reception. Some of these methods required the use of one or more coupling tubes, and usually had the disadvantage that the automatic 'frequency control (AFC) voltage appeared as a balanced-to-ground voltage instead of the more desirable potential against ground.` o

Accordingly, it may be stated that it is one of (Cl. Z50-271) ganizations whereby my ini/ention'may be carried into effect.

In the drawings:

Fig. 1 shows one arrangement embodying the invention, q

Fig. 2 shows a modication of theembodiment of Fig. 1, a dierent type of discriminator network being employed, V

Fig. 3 shows a further modification wherein still another form of discriminator network is employed, i

Fig. 4 illustrates a further modification wherein the diierential detector is used to drive a pushthe main objects of this invention to provide a l,

simple type of differential diode detector circuit which eliminates the necessity of using extra tubes and produces the desired type of AFC potential; and which can be utilized for either of FM or AM (amplitude modulatedl carrier wave) reception by the expedient of adjusting a simple switch mechanism. p

Another important object of this invention is to provide a diierential diode detector circuit which is so arranged that, when fed from a fre- I quency discriminator network, terminals are available for detected AM voltage, FM voltage; automatic volume control (AVC) and AFC voltages, and these terminals being independent of each other so -that amplitude andfrequency modulated carrier waves may be detected simultaneously and AFC .and AVC potentials be conindicated diagrammatically several circuit orpull amplifier,

Fig. 4a illustrates a; modication of a portion of the differential detector circuit of Fig. 4.

Referring now to the accompanying drawings, wherein like reference characters in the different figuresv designate similar circuit elements, the circuit arrangement of Fig. l illustrates a differential diode detector circuit which may be utilized in a. superheterodyne receiver of the type capable of receiving FM or AM waves. It is not'believed necessary to describe the networks prior to the input transformer of the detector circuit, because those skilled in the art are fully acquainted with such receiver networks. It is believed suillcient to point out that in the case of FM reception, and assuming the FM reception to cover 42-50 megacycles (Mc.) the receiver would utilize a converter network to reduce the selected FM` waves to aylower frequency. The operating intermediate frequency (I. F.) may be of the order of 4.3 Mc., and theA I. F. waveswould be amplified in several stages of I. F. amplification followed by any well known form of amplitude modulation limiter device. The pass band of the couplingnetworks would depend upon the center frequency deviation range, and, of course, the pass .band of the various coupling networks would be suiliciently wide to pass the entire center frequency deviation range. Assuming that wide band FM waves are employed, it will be understood that a pass band of substantially to 200 kilocycles -(Kc.) will vbe utilized in eachV lsecondary resonant circuits I0 and Il. Each of them is provided` with a damping resistor to provide a suiliciently wide passband. These secondary circuits l'and II are mistuned with respect to the center frequency of the applied FM waves. The mistuning is in opposite senses, but in equal frequency amounts. Thus, tuned circuit l could be tuned to a certain number of kilocycles above the center frequency, while circuit II would be tuned the same number of kilocycles below the center frequency. This type of discriminator network is well known in the art, and

it is not believed necemary to describe it in any further detail.

This type of discriminator converts applied FM waves to AM waves with modulation envelopes 180 degrees out of phase. These modulation envelopes are detected by diodes 2 and 3 which make the detected outputs available across resist'ors 4-5 and 6 1. Resistors 4 and 5 are arranged in series between the cathode of diode 2 and the low potential side of tuned circuit I0, the I. F. by-pass condenser 8 being connected in shunt across resistors 45.l Simultaneously, resistors 1 and 6 are arranged in series relation between the cathode of diode 3 and the low potential side of tuned circuit II, the I. F. by-pass condenser 9 shunting vresistors 6 1. The junction of resistors 4 and -5 and the junction of resistors 6 and 1 are directly connected together, and the cathode end of resistor 1 is established at ground potential. The manner in which these sincefit is a summation of the voltages across resistors 1 and 5. The second point from which to derive AVC voltage is designated AVC', and this output lead is connected to the anode end oi' resistor 6. 'I'his output point AVC' gives a control potential corresponding to the voltage from one detector only, since it is the sum of the voltages across resistors 1 and 6 which are both in the same circuit with diode 3. The choice of which of these two points to use to obtain the AVC potential depends upon the exact nature of the discriminator. which feeds the diodes. For most purposes there will be little difference 'oetween the types of AVC action obtained from the resistors are connected together makes the detected FM voltage available across resistors 4 and 1.

That is to say,'the FM voltage is taken off at the cathode end of resistor 4. Hence, the letters FM are applied to a terminal connected directly to the cathode end of resistor 4. It will be noted that the output of the two resistors 4 and 1 is reversed in phase so that the proper phase reversal is accomplished for the addition of the detected FM voltage. The slow variations of frequency, corresponding to carrier or local oscillator frequency drift rates, also appear across these resistors 4 and 1 which are available for AFC purposes. Therefore, the AFC connection is made to the FM voltage output lead. As is well known to those skilled in the art, the AFC bias may be employed to control the local oscillator of the receiver so as to vary its frequency in a sense to insure, 'or maintain, the correct value of I. F.

If the receiving system is used to receive AM waves, then it is merely necessary to connect an output terminal, designated AM, to the anode end of resistor 5. 'I'he AM voltage output is taken from across resistors 5 and 1. It will be noted that these two resistors 5 and 1 are connected so that they both have their ends which are towards the diode plates at high potential, and the combination of the detected rAM Voltage outputs is in phase. The normal adjustments of the resistors are to make them all equal in order to effect the balanced conditions for both AM and FM reception. It is to be noted that the by-pass condensers 8 and 9 will by-pass the I. F. currents fed to the discriminator, but allow the modulation frequencies to pass.

There are two points from-'which AVC potentials may be obtained. The rst point is from the same point that the AM voltage output is obtained, and, hence, the letters AVC are applied to a lead connected to the AM voltage output lead. This point gives a potential corresponding to alsummation of voltages from both detectors,

two. points. 'I'hose skilled in the art are fully acquainted with the manner of using the AVC bias. It is only necessary to point out that the AVC bias ils applied to the ampliers -prior to the differential detector so as to vary the gain of these ampliers in a sense to compensate for carrier fading.

Any adjustable switch element I2 is provided so as to select either FM or AM voltage depending upon the type of reception which is had. 'I'he adjustable element I2 is connected to the input electrode of the following audio frequency amplier, and the element I2 may bev adjusted to contact either the FM or AM output terminal of the differential detector. It is pointed out that when receiving AM waves with this system the AM waves will pass through the receiving system in the usual manner, it being only necessary that the carrier frequency of the AM waves b'e of a. value equal to the tuned frequency of the pri- 'mary resonant-'circuit I', or stated more generally, the carrier frequency of the AM should be located in the pass band of the transformer I. Simultaneous derivation of detected FM and AM voltages could be taken olf in such cases where the carrier at the transmitter is simultaneously frequency modulated by one type of signal and amplitude modulated by another type of signal. In such a case, of course, adjustable switch element I2 would be replaced by a pair of leads connected to the respective FM and AM output points.

When the common carrier is frequency and amplitude modulated a small degree of amplitude modulation (probably not over about 50 percent) would be used for the AM signals, and the carrier is frequency modulated in the normal manner. Another way of detecting this type of modulation would be to divide the signal into two branches one of which is a frequency modulation detector and the other an amplitude detector. With such separate branches, the FM detector may include a limiter to more completely remove the amplitude modulation, but with the system shown here, the balance of the detectors would be depended upon to cancel the amplitude modulation from the FM output. No limiter would be used prior to the detector in Fig. 1 for AM or simultaneous FM and AM reception.

In Fig. 2 the diodes 2 and 3 are shown connected to a different type of discriminator input circuit. The discriminator in this case is of the type disclosed by S. W. Seeley in U. S. Patent 2,121,103, granted June 2l, 1938` The primary circuit is again tuned to the center frequency of the applied FM waves. Instead of using a single tuned secondary circuit, two secondaries I3 and I4 are employed, the condenser I5 shunting the secondaries and tuning them to the center frequency of the applied waves. Th'e high potential side of circuit I is connectedby a direct current l anodes of diodes 2 and 3.

blocking condenser I6 to the cathode end of coil I4, while the direct current blocking condenser I1 connects the high potential side of circuit I' to the cathode end of coil I3.v Y are again arranged in series between the cathode of diode 2 and the `low potential end of coll I3, a choke coil 5 being connected in series with resistor 5. Thecathode end of resistor 1 is at ground po "tential and a choke coil 6' is in series with'A resistor 6. The resistors and 6 are connected in Resistors 4 and series between the cathode diode 3 and the vlow l potential end of coil I4. The two secondaries I3 and I4 are used in place of the usual mid-tapped secondary winding in order tb isolate the rectified currents in the two resistors adjacent the chokes. It is not believed necessary to describe the manner in which the discriminator circuit operates in Fig. 2 to produce the detected FM voltage between the cathode end of resistor 4 and ground. It will be noted that the various output voltage connections are made to substantially the same point as in the case of Fig. 1.

In Fig. 3 there is shown an arrangement wherein the discriminator input circuit is of the type disclosed in my U. S. Patents 2,085,008 and 2,156,374, and, also,'disclosed in my application Serial No.I 355,592, filed September 6, 1940.'

While this detector is primarily a phase modu- 'lation (PM) detector, it will detect FM waves if the degree 'of modulation is low enough. In this type of circuit` the transformer I has its primary circuit I and secondary circuit I" constructed and arranged so as to have apass band equal to the channel width of the applied waves. Each of I and I is tuned substantially to the carrier is possible, and AFC bias is" simultaneously produced. 'I'he usual procedure, however, is the alternative reception of phase or amplitude modulated waves by means of switch I2. Hence, it will be understood that the circuit arrangement in Fig. 3 is useful for receiving phase modulated o r amplitudemodulated carrier waves. The letters PM designate the detected phase modulated voltage output terminal, and it will be noted that the terminal isconnected tothe cathode end of resistor 4, as in the case of Figs. 1 and' 2 where the FM output terminal is made to the same point. When FM waves are received by this circuit of Fig. 3, the S-shaped curve of Fig. 9

of my I. R. E. paper is depended upon for detection.

In Fig. 4 there is shown the arrangement of Fig. 1 constructed to drive a push-pull audio amplifier. There is shown the audio amplifier tubes 30 and 3| each having their signal input grids arranged for push-pull operation, and the plate circuits are arranged for push-pull operation as well. The signal grid 32 of tube 30 is connected through the 'audio coupling condenser 33 tothe adjustable element 'I2 which may be adjusted to select eitherof the detected FM or AM voltage output terminals. The junction of resistors' 4 and 'I is atl ground potential, while the grid 40 of tube 3| is connected by the audio coupling condenser 4| `to the cathode end of resistor It will be noted that the usual method of reversing the phase of combination when s uch a push-pull audio amplifier is used makes use of a reversing switch between one of the amplifier frequency. Aepiezoelectric crystal 20, tuned to of the `manner of operation of the discriminator, r

reference being made to my aforesaid patents and application for a detailed explanation, as well as to my paper Communication by Phase Modulation in Proceedings of I. R. E. for February, 1939. It is suflicient to point out that the crystal functions to rotate the phase of the vector resultant of the modulation side bands of the applied PM waves so that the resultant vector of the modulation` side bands is in phase with the carrier. By means of this phase rotation action, the PM waves are converted to AM waves which are rectified by the differentially connected diode detectors. Specifically, resistors 4 and 5 are arranged in series with a resistor 25 between the` anode and cathode of diode 2. Resistors 1 and 6 are arranged in series relation with resistor 26 between the anode and cathode of diode 3. Resistors 25 and 26 provide direct current cricuits for the diodes, and, also, act to divide the crystal output .into two portions.

plates and the output transformer. 'I'his requires an output transformer with both plate windings separated instead of the'mid-tapped type shown in Fig. 4. The prior arrangement, also, has the disadvantage that for the amplitude modulation connection the tubes are no longer connected in push-pull, so'that the benet of the direct current flux balance is not obtained. In the circuit `arrangement .of Fig. 4 it is possible to use a single-pole, double-throw switch to change from amplitude modulation to frequency modulation reception'. l

Here, again, the various output voltage connec-` of phase modulation and amplitude modulation 1n me `circuit of Fig. 4a, it is shown how the separateresistors 4-5 and 1-6 of the two diodes 2 and 3` may be by-passed individually. This type of by-passing sometimes is advantageous since all pointsof .the resistors are then bypassed, and the mid-tapof the upper diode resistors is not left unby-passed by the presence of the lower diode resistors between the mid-tap and ground.

While I have indicated andV described several systems for carrying my invention into effect, it

will be apparent to one skilled in the art that my `invention is by no means limited tothe par ticular organizations shown and described, but that many modiilcations may be made without departing from the scope of my invention, 4as sei forth in the appended claims.

What I claim is:

`1. In combination, a rst diode having a modu lated carrier wave input circuit, a pair of resistor elements arranged in series relation beltween thel cathode `of said diode and one side of said input circuit, a second `diode provided with a modulated carrier wave input circuit, a

. second pair of resistors arranged in series rela-- tion between the cathode of the second diode and one side of its input circuit, a direct current voltage connection between the junction of said `first pair of resistors and said second pair of resistors, means for applying to said diode input circuits modulated carrier waves of variable center frequency or modulated carrier waves of constant carrier frequency and variable amplitude, like resistors of said two pairs of resistors being differentially connected to provide thereacross modulation Voltage corresponding to the aforesaid center frequency variations, means for deriving said modulation from across said differentially connected resistors, and additional means connected to the anode end of one of said pair of resistors for deriving modulation voltage corresponding to said modulated waves of variable carrier amplitude.

2. In combination with a first diode having a modulated carrier wave input circuit and a pair of resistors arranged in series relation between the anode and cathode of the diode, a second diode having a modulated carrier wave input circuit and a second pair of resistors arranged in series relation between the anode and cathode of Ithe second diode, means applying to said input circuits modulated carrier waves which are alternatively frequency or amplitude modulated, means establishing the cathode end of one of said pair of resistors at ground potential, means connecting in series relation between the cathodes of said diodes nke resistors of said two pairs of resistors, means for connecting a detected frequency modulation voltage output terminal to the ungrounded cathode end of said pair of like resistors, and a separate detected amplitude modulation voltage terminal connected to the anode end of one of said pair of resistors.

3. The method of combining the outputs of two detectors so that the differential and additive outputs are available simultaneously, in which each output resistor is divided into two sections with the joining point of one pair of sections connected to the joining point of the other pair of sections, and obtaining said differential and additive outputs from the resulting series combinations of said sections.

4. In a balanced frequency modulation detecting system, the method of making an amplitude modulation detected output available which includes joining the mid-points of the output resistors of each detector and taking the two outputs from the resulting series combinations of resistors.

5. In a balanced frequency modulation detecting system, the method of obtaining the detected frequency modulation output, automatic-frequency-control potentials, detected amplitude modulation output, and automatic-volume-control potentials, which includes joining points near the middle of each detectorcutput resistor and obtaining said outputs and potentials from the resulting series connections of resistors.

6. In combination, a irst diode having afrequency and amplitude modulated carrier wave input circuit, a pair of resistor elements arranged in series relation between the cathode of said diode and one side of said input circuit, a second diode provided with a modulated carrier wave input circuit, a second pair of resistors arranged in series relation between the cathode of the second diode and one side of its input circuit, a connection between the junction of said iirst pair of resistors and said second pair of resistors, like resistors of said two pairs of resistors being differentially connected to provide thereacross modulation voltage corresponding to center frequency variations of the modulated carrier waves, means for deriving said modulation from across said differentially connected resistors, and additional means connected to the anode end of one of said pair of resistors for deriving modulation voltage corresponding to amplitude variation of said modulated waves.

'1. In combination with a rst diode having a modulated carrier wave input circuit and a pair of resistors arranged in series relation between the anode and cathode of the diode, a second diode coupled to said modulated carrier wave input circuit, a second'pair of resistors arranged in series relation between the anode and cathode of the second diode, means applying to said input circuit modulated carrier waves which are alternatively phase or amplitude modulated, means establishing the cathode end of one of said pair of resistors at ground potential, means connecting in series relation between the cathodes of said diodes like resistors of said two pair of resistors, means for connecting detected phase modulation voltage output terminal to the ungrounded cathode end of said pair of like resistors, and a separate detected amplitude modulation voltage terminal connected to the anode end of one of said pair of resistors.

8. In combination with a first diode having a modulated carrier wave input circuit and a pair `of resistors arranged in series relation between the anode and cathode of the diode, a second diode having a modulated carrier wave input circuit, and a'second pairl of resistors arranged in series relation between the anode and cathode of the second diode,` means applying to said input circuits modulated carrier waves which are alternatively frequency or amplitude modulated, means connecting in series relation between the cathodes of said diodes like resistors of said two pairs of resistors, means grounding the junction of said like resistors, means for connecting detected frequency modulation voltage output terminals to the ungrounded cathode ends of said pair of like resistors, and a separate detected amplitude modulation voltage terminal connected to theanode end of one of said pair of resistors.

9. In combination, a rst diode having a modj ulated carrier wave input circuit, a pair of resistor elements arranged in series relation between the cathode of said diode and one side of said input circuit, a second diode provided with a modulated carrier wave input circuit, a second pair of resistors arranged in series relation between the cathode of the second diode and one side of its input circuit, a connection between the junctions of said first pair of resistors and said second pair of resistors, means for applying to the input circuit of each of said diodes modulated carrier waves, like resistors of said two pairs of resistors being differentially connected to provide thereacross a first modulation voltage corresponding to frequency variations of the carrier waves, means for deriving said modulated carrier wave input circuit, a pair of resistors arranged in series relation between the anode and cathode of the diode, a second diode utilizing said modulated carrier wave input circuit, a piezo-electrical crystal, tuned to the frequency of the said input circuit, coupling said anode and cathode of the second diode, means' applying to said input circuit modulated carrier waves which are alternatively frequency or amplitude modulated, means establishing the cathode end of one of said pair of resistors at ground potential, means connecting in series relation between the cathodes of said diodes like resistors of said two pair of resistors, means for connecting a detected `frequency modulation voltage out'put terminal to the ungrounded cathode end of saidA pair of like resistors, and a separate detected amplitude modulation voltage terminal connected to the anode end ofone of said pair of resistors.

11. In combination, a pair of detectors arranged so that the differential and additive outputs are available simultaneously, each detector having an output resistor divided into two sections, the junction of one `pair of sections connected to the junction of the other pairf of sections, and means obtaining said differential and additive output from the `resulting series combinations of said sections.

12. In combination, a first diode having a frequency and amplitude modulated` carrier wave input circuit, a pair of resistor elements arranged in series relation between the cathode of said diode andone side of said input circuit, a second diode provided with a modulated carrier wave input circuit, a second pair of resistors arranged in series relation between the cathode of the second diode and one side of its input circuit, a connection between the junction of said iirst pair of resistors and said second pair of resistors, like resistors ofvsaid two pairs of resistors being differentially connected to provide thereacross modulation voltage corresponding to frequency variations of the modulated carrier waves, means including a push-pull amplifier for deriving said modulation from across said differentially connected resistors, and additional means, connected to the anode end of one of said pair oi resistors,

` for applyingv modulation voltage corresponding 13. In` combination with a first diode having a` modulated carrier wave input circuit and a pair of resistors arranged in series relation between 'the anode and cathode of the'diode, a. second pair of resistors arranged in series lrelation between the anode and cathode of the seconddiode,

means applying to said 'input circuit modulated carrier waves, means establishing the cathode end of one of saidpair of resistors at relatively invariable potential, means connecting in series relation between the cathodes of said diodes like resistors of said two pair of resistors, means for connecting an output terminal for `frequency variable modulation to the ungrounded cathode end of said pair of like resistors, and a separate amplitude modulation output terminal connected to the anode end of one of said pair of resistors.

14. In combination with a. iirst diode having a modulated carrier wave input circuit and a pair of resistors arranged in series relation between the anode and cathode of the diode, asecond diode having a modulated carrier wave input circuit, and a'second pair of resistors arranged in series relation between the anode'and cathode of the second diode, means applying-to said input circuits modulated carrier waves, means conneoting in series relation between the cathodes of said diodes like resistors of said two pair of resistors, means grounding the junction oi said like resistors, means for connecting detected frequency modulation voltage output terminals to the ungrounded cathode ends of said pair of terminals.

15. Inl a detector of signal-modulated carrier energy, a frequency-responsive network compris- 4ing two circuits having frequency-response chary acteristics of the same type but sloping in opposite senses with frequency over a predetermined range of frequency deviation of a carrier signal applied thereto, a. pair of rectiiiers individually coupled to said circuits, a load impedance in circuit with one of the two circuits and one rectifier,

a second separate load impedance in circuit with the second circuit and the second rectifier, fractional portions of said impedances being in series` relation, and means for separately deriving from diierent points o1 said load impedances difference and summation components respectively of separately rectied outputs of said rectitlers.

MURRAY G. CROSBY. 

